Restore affirmative responses for "audio/mp4" when avc1 is supported (re-land r241165).

This case was broken by r221932/r222995.

BUG=329065
TEST=Updated browser_tests.

Review URL: https://codereview.chromium.org/117763002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241498 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241498